The present invention relates to communication systems and more particularly to pulse doppler radar systems having large bandwidth with large dynamic range.
The maximum bandwidth of communication systems has generally been limited to a spurious-free frequency band over which transmitted communication signals can be received for processing by a receiver, and transmitted spurious signals (spurs or birds) and receiver generated birds are substantially excluded from receiver processing. As a result, communication systems have generally had a smaller bandwidth than that which is desirable for better system operation.
In airborne pulse doppler radar systems, maximum bandwidth has generally been design limited to about 3 or 4 GHz. However, much wider bandwidth is desirable to provide greater flexibility to radar operators in identifying targets or avoiding jamming.
State-of-the-art airborne radar systems, or ground based radar systems having a clutter tracking function, are mechanized so that all stable local oscillator (STALO) frequency signals, except one, are harmonically related to a single master oscillator. Hence, spurious mixing products are also harmonically related to the master oscillator frequency. However, the one exception is a clutter tracking frequency signal which is unrelated harmonically to the master oscillator signals, and, accordingly, has caused problems that have limited system bandwidth in the prior art as described above. Thus, one or more harmonics of the clutter tracking signal can fall near a transmit frequency signal or near a receiver LO1 frequency signal and thus can be processed by the receiver and appear to be one or more false targets.
A clutter tracking function is required in the operation of airborne pulse doppler radar systems, and may be required in stationary ground-base pulse doppler radar systems, to enable ground clutter returns to be tuned for removal from receiver processing. A clutter frequency signal is generated to correct for doppler shift of ground clutter returns and such use of the clutter frequency signal is referred to as dithering.
The clutter function is typically provided in the transmitter. However, the clutter frequency signal can be generated by dithering either the transmit frequency signal or one of the receiver LO frequency signals so that radar ground clutter returns are positioned in a receiver notch filter and thus cannot be processed as false radar target returns.
Ground based, coherent doppler or moving target indicator (MTI) radars may also require a clutter oscillator, and such radars have nonetheless had limited frequency ranges of operation for reasons similar to those described for the airborne pulse doppler radar systems.
Even in pulse doppler radar systems which have no dithering function, there can be many regions in the receiver first mixer and in the transmit mixer which have bird problems. The use of a dithering function, as described above, makes the bird problems much more severe and harder to resolve. System requirements determine how difficult particular bird problems are in each system design.
When prior art attempts have been made to provide larger radar bandwidths, the problem of dealing with the clutter tracking harmonics has been especially severe because many regions of the total LO1/transmit frequency band are harmonically related to the clutter track frequency. In the prior art, spurious or bird signals, which stem from the clutter oscillator, fall in these regions without blockage. This bird problem is the principal reason that bandwidth has been limited in prior art pulse doppler radar systems.
Direct frequency (DF) synthesizers have been available to generate frequency signals over very broad ranges, such as 3 GHz to 20 GHz. A direct frequency synthesizer suitable for wideband operation is disclosed in U.S. Pat. No. 5,166,629, entitled "Direct Frequency Synthesizer", and issued to G. Watkins et al. on Nov. 24, 1992. However, such DF synthesizers have not been able to be used to greatest advantage in prior art pulse doppler radar systems since such radar systems have had a limited bandwidth as described above.
Further, in general, communication systems have been similarly limited in bandwidth as a result of the presence of birds in many regions of the possible total transmit frequency band. As in the specific case of pulse doppler radar systems, other communication systems may or may not employ dithering, and in either case be characterized with significant bird problems that limit system bandwidth unless they are resolved.
Spread spectrum communication systems, for example, have a need for use of an ultra wide band of frequencies to send high volumes of data. Especially under congested band conditions, broad band operation is needed to provide flexibility for operating at frequencies where there is least interference and optimum atmospheric conditions. Further, to avoid unwanted interception of sensitive communications by an undesired party, frequency is desirably changeable over a large bandwidth to frequencies known only by the intended parties. In order to avoid jamming by an undesired party, frequency may also be changed rapidly, desirably over a large band of frequencies.
While there are certain significant differences, there is some similarity between broad band radar systems and spread spectrum communication systems. To obtain a stronger target return, a radar system uses several frequencies so that degradation due to atmospheric conditions and target resonance with respect to radar wavelength can be avoided. The radar system selects frequencies where there is less interference from any neighboring radars. Similarly, the communication system selects different frequencies to adjust to atmospheric and sunspot conditions and to interference conditions.
Receivers in both systems are susceptible to the generation of undesired intermodulation products in the first mixer. In the case of radar systems, such products are generated by strong ground clutter returns or jammers or other radars. In the case of spread spectrum communication systems, such products are generated by jammers or other communication systems.
Both radar systems and spread spectrum communication systems require a receiver and a transmitter. When manufactured by a common manufacturer, both systems are usually designed with as much common hardware as possible to optimize cost and performance. The radar receiver employs a variable frequency LO for its first mixer, and the communication receiver employs a variable frequency LO for its first mixer. The radar transmitter employs a variable output frequency which tracks its receiver, and the spread spectrum communication system has an identical requirement. In the process of generating the transmit frequency for either system, spurious mixer regions are encountered as the receiver first LO signal is mixed with an offset signal having a frequency which is equal to the frequency of the receiver first IF, to generate the transmit signal with a frequency that is variable over a broad band.
The large ground clutter returns associated with airborne radars create a unique problem which must be overcome. These returns overload the signal processing circuits if they are not filtered out by dithering a receiver LO frequency or the transmit frequency so that such returns fall in a narrow notch filter. Similar problems of a less severe nature can be encountered in ground based radars from ground returns from trees which blow in the wind, and dithering such returns into a notch filter can be required.
In communication systems, it may be necessary to offset the received frequency from the transmit frequency by several Khz to notch out a strong interfering signal. Hence, a similar dithering function is required. Most radio amateur communication transceivers have such a feature.
Whether a dithering function is required in air or ground based radar systems or other communication systems which are attempted to be operated over a broad band of frequencies, there are bad spurious mixing regions in both the transmitter mixer and the receiver first mixer. Accordingly, prior art radar and other communication systems have generally been limited in capability to handle birds in such spurious regions and have therefore been disadvantageously limited in bandwidth.